This invention relates to the treatment of "sour" petroleum and coal liquefaction oils containing hydrogen sulfide and other organosulfur compounds such as thiols and thiocarboxylic acids, and more particularly, to improved methods of treating such streams by using choline base.
Petroleum and synthetic coal liquefaction crude oils are converted into finished products in a fuel products refinery, where principally the products are motor gasoline, distillate fuels (diesel and heating oils), and bunker (residual) fuel oil. Atmospheric and vacuum distillation towers separate the crude into narrow boiling fractions. The vacuum tower cuts deeply into the crude while avoiding temperatures above about 800.degree. F. which cause thermal cracking. A catalytic cracking unit cracks high boiling vacuum gas oil into a mixture from light gases to very heavy tars and coke. In general, very heavy virgin residuum (average boiling points greater than 1100.degree. F.) is blended into residual fuel oil or thermally cracked into lighter products in a visbreaker or coker.
The residue or bottoms from the distillation can either be coked in delayed coking drums at temperatures between 900.degree. to 930.degree. F. to produce coke and distilled overhead products or can be shipped and sold directly to be used as fuel in boilers on ships or in power or steam plants.
Industrial fuel oils can also consist of one or more of the following products derived from petroleum: vacuum tower bottoms, catalytically cracked light or heavy gas oils and catalytically cracked clarified oil. A No. 6 fuel oil is an example of such a product. It will typically have a boiling point of 660.degree. F., a specific gravity greater than 1.0.degree. and 45.degree. F. pour point. These products can contain significant amounts of H.sub.2 S. However, substantial amounts of hydrogen sulfide, as well as mercaptans and organosulfides, may be found in the atmospheric and vacuum distillation tower bottoms, which may be blended into gas oils and fuel oils. When sour heavy fuel oils are to be used as fuel in industrial boilers or in electric utility steam plants or elsewhere, the presence of large amounts of H.sub.2 S in the atmosphere above or associated with the oil during transit in barges or ships, or in storage in refinery tanks or user tanks, constitutes a hazard to personnel who are in the vicinity of the storage or transport facilities. Typically, the H.sub.2 S specification for such fuel oils is 100 ppm or less.
Heavy fuel oils are very viscous and in order to pump them from one location to another, it is often necessary to heat them to temperatures exceeding 230.degree. F., the decomposition temperature of choline base. The temperature to which the heavy fuel oil is heated is dependent, among other factors, upon the rate of heat loss of the conduit through which the fuel oil is pumped, the velocity of flow through the conduit, and ambient temperatures of the environment through which the conduit passes. In an example of the West Coast Refinery, it is necessary to heat the heavy fuel oil to a temperature of about 400.degree. F. to pump it to a terminal loading facility some 20 miles from the refinery. Heating heavy sour fuel oils increases a hydrogen sulfide atmosphere which may be hazardous to operating personnel.
The prior art relating to the treatment of sour petroleum oils includes methods in which choline base has been employed to treat sour heavy fuel oils to maintain the hydrogen sulfide content in the atmosphere above or associated with such oils at levels within acceptable limits to avoid health hazards to personnel, as disclosed in U.S. Pat. No. 4,867,865. (Choline base also has been used to treat gasoline and other motor fuels to remove organosulfur compounds such as thiols, thiolcarboxylic acids, disulfides and polysulfides, as disclosed in U.S. Pat. No. 4,594,147.) As disclosed in U.S. Pat. No. 4,867,865, a choline base has been used in the past to reduce hydrogen sulfide content of atmospheres above sour heavy fuel oils from as high as 5,000 ppm down to 100 ppm or less by treating such sour oils at temperatures below the decomposition temperature of choline base, which is about 230.degree. F.